1. Field of the Invention
The present invention relates to a neutron chopper used for a neutron scattering experiment device which radiates neutrons on a specimen, and analyzes an internal structure and the like of the specimen based on scattered neutrons.
2. Description of the Related Art
The neutron scattering experiment device radiates neutrons on a specimen, and observes neutrons scattered by the specimen, thereby analyzing physical properties (internal structure) of the specimen. In the neutron scattering experiment device, neutrons generated in a pulse form at a neutron generation source are led to the specimen by a beam transport system designed to transport neutrons at a low loss by means of neutron guides (such as super mirrors). In the course thereof, a time interval of the beam is properly shaped and selected by means of a neutron chopper or the like. The neutrons made incident to the specimen are scattered at a specific angle and at a specific velocity according to an arrangement and a mode of movement of atoms and molecules in the specimen and detected by a neutron detector. The energy of the scattered neutron is determined by measuring the time of flight of the pulsed neutron beam, and is analyzed along with the angular dependency of the scattering intensity, and experiment results are extracted consequently.
In a nuclear spallation neutron source as the neutron generation source, in a moment when protons collide with liquid mercury target, high-speed neutrons are generated, and are transmitted instantly. The observation by means of the neutron scattering experiment device is carried out at a high precision and at a high sensitivity, and, thus, the high-speed neutrons generated at the predetermined interval constitute a background source, resulting in a cause of interference on the observation.
“Development of Neutrons T0 chopper”, Ryuji Ohkubo and four other persons, 7th High Energy Accelerator Research Organization Mechanical Engineering Workshop Reports, available on the Internet at URL: http://ilc.kek.jp/MechWS/2006/, discloses a neutron chopper (so-called T0 chopper) as a device which blocks the high-speed neutrons. This neutron chopper includes a rotating body provided with a metal hammer having a mass enough for blocking neutrons having an unnecessarily high energy of several hundreds meV, and a mechanism which rotates the hammer highly precisely in synchronism with the generation of the pulsed neutrons. Specifically, a hammer, which is made of Inconel X-750 (registered trademark), which is a high-Ni material for blocking neutrons, is provided integrally with a rotor in a vacuum container. Then, a power from a motor provided outside the vacuum container is transmitted to the rotor of the neutron chopper via a magnetic seal unit, thereby rotating the rotor and hammer. By blocking a beam line with the hammer only in a neighborhood of the time origin (t=0) at which the high-speed neutrons are generated, the high-speed neutrons are prevented from being transmitted to the downstream of the experiment device. Neutrons required for the analysis experiment are low in energy and are slow in the flight speed, and, thus, reach the neutron chopper later than the high-speed neutrons. Therefore, by adjusting the timing of the rotation of the hammer, it is possible to prevent the hammer from blocking the beam line at the time of the arrival, and it is thus possible to remove only the unnecessary high-speed neutrons constituting a background source without preventing the transport of the necessary neutron beam.
On this occasion, when neutrons pass through the atmosphere, the neutrons collide with molecules of the air, are scattered, and are attenuated, and it is thus necessary to provide neutron guides in front and rear of the neutron chopper as close thereto as possible. However, in the neutron chopper described in “Development of Neutrons T0 chopper”, the motor and the mechanism for transmitting the power of the motor are provided outside the housing forming the neutron chopper, resulting in a configuration in which it is hard to arrange the neutron guides close to the housing, which poses a problem.
Moreover, the neutron chopper described in “Development of Neutrons T0 chopper” is configured such that the motor in the atmosphere transmits the power to the hammer in the vacuum, and, thus, tends to generate a vacuum leak, which poses a problem.
Further, this neutron chopper requires a large number of devices such as a magnetic seal unit which restrains the vacuum leak, a coupling which transmits the power from the motor to the hammer, and a timing belt, resulting in an increase in the cost, and an increase in the size of the neutron chopper, which pose a problem.